High-pressure accumulator injection systems, also known as common rail injection systems, are distinguished from conventional injection systems in that the injection pressure can be generated independent of the engine speed. The decoupling of the pressure generation and injection is achieved with the aid of an accumulator, in which fuel is stored under high pressure. The high pressure in the accumulator is created by means of a high-pressure pump. Fuel from the accumulator is used to supply an injection valve and also a control chamber, by means of which a valve needle of the injection valve is controlled. A control piston is also fitted in the control chamber in such a way that it can slide, with one end of the piston being connected to the valve needle and pressure being applied to its other end in the control chamber. The pressure in the control chamber is supplied from the pressure in the accumulator via a connecting line. The control chamber is connected to a valve to release the pressure. Furthermore, an inlet throttle is positioned between the accumulator and control chamber and an outlet throttle between the control chamber and valve to guarantee a predetermined pressure build-up or reduction in the control chamber after the closing or opening of the valve respectively.
The outlet throttle is designed so that the cavitation transition point, i.e. the backpressure, which if undershot means that the flow through the throttle can no longer be increased due to cavitation and therefore a backpressure is felt downstream of the throttle regardless of the direction of flow, is as high as possible. This causes cavitation to occur at the outlet throttle with the valve open (low backpressure) and flow through the throttle, and thus movement of the control piston becomes independent of the cross-sectional area of flow of the valve.
The predetermined pressure build-up/pressure reduction in the control chamber creates a controlled movement of the control piston and the valve needle connected to it. Controlled in this case means that the time point of the start of movement when opening and closing, and also the speed of movement itself, can be predetermined by the size of the cross-sectional areas of the control piston and valve needle to which pressure is admitted as well as by the fuel pressure in the accumulator and the flow characteristics of the throttles, particularly flow resistance and cavitation point. The reproducible injection of defined amounts of fuel with high precision thus demands a high degree of accuracy in the manufacture of control pistons and throttles. The relatively large cross-sectional areas of control pistons can be very accurately manufactured but the production of throttles with low production tolerances on the other hand demands very high expenditure, as explained in the following.
The throttles used for injection devices according to the prior art are in the form of cylindrical cross-sectional convergences in the flow path between the control chamber and accumulator or between the control chamber and valve. Such conventional throttles typically have a length of approximately 1 mm and a typical throttle passage diameter of 0.3 mm. The throttle passage is, for example, produced by drilling or by electrochemical erosion. The length of the throttle itself is of minor significance with regard to the flow properties of the throttle. The flow properties of the throttles are, however, not only determined by the diameter of the throttle passage, but also by any taper, the shape of inlet and outlet edges and the surface finish of the throttle passage. The setting of the flow resistance, that determines the function of the throttle, to the set value is achieved by rounding the inlet edges using hydroerosion. Throttles with fine tolerances and uniform quality with regard to flow parameters can thus be produced only at high cost. In practice, the throttle manufacturer must also take account of a correspondingly high rejection rate.